The role of anti-group A streptococcal carbohydrate (SACHO) immunity in producing some of the pathologic sequelae seen following group A streptococcal infections has long been debated. The proposed research aims at developing an animal model that will allow a careful analysis of the humoral immune response to SACHO. Such things as genetic influences on the magnitude of the precipitin and non-precipitating antibody response and the inheritance of idiotypic determinants will be investigated. The role of the thymus in modulating these responses will be studied. By comparing our own lines of rats, bred for high and low precipitin responses, with inbred rats of known histocompatibility loci, it may be possible to correlate the various parameters of the humoral response with known AgB loci.